Nuclear reactors are used to generate steam or gas to drive turbo-generator sets in nuclear power plants, to generate electric power.
In the nuclear reactor pressure vessel is located the nuclear core, which is surrounded by a moderator, which consists as a rule of light water or garphite. This moderator is necessary in the thermal reactions, to obtain a nuclear fission process and, in the case of light-water reactors, is at the same time the reactor coolant which evaporates either in the reactor directly or generates this steam by passing the hot water coolant through the heat exchanger of steam generators.
In "fast" nuclear reactors, the so-called "breeders," the coolant consists as a rule of sodium.
To control the nuclear process, the coolant, when water, is partly pumped by forced-circulation pumps with different flow rates, particularly in the case of light-water reactors. This so-called "circulation control," however, is not sufficient in all cases and particularly not for shutting the nuclear reactor core down completely.
In principle, there are in the reactor core, absorber rods which can be run in and out of the core and which, depending on their positions, capture more or fewer of the fission neutrons required for the core's chain reaction, to intervene thereby in the nuclear process and contribute to a change of the thermal power output delivered; but these absorber rods, or control rods, are also capable of shutting down the reactor core completely.
The absorber rods are moved by so-called control rod or control assembly or absorber rod, drives, which are located either outside or inside the reactor pressure vessel. These drives, together with their associated absorber rods, have the purpose of extending the range of the coolant circulation control, to set power ranges for the circulation control and to bring about control of the burnup for the nuclear fuel.
In case of trouble, on the other hand, the nuclear reactor must be shut down quickly by the drives. For safety reasons, it is therefore necessary that a faster absorber rod moving system be coupled with the more slowly operating moving system used for the purposes mentioned above. Both systems must be operative independently of each other for safety reasons and therefore consist as a rule, of a hydraulic drive for the fast shutdown and a mechanical threaded spindle and nut drive for the slower movement of the absorber.
After a fast shutdown, it is important particularly for boiling-water reactors, in which the absorber rods are shot upwardly into the core against the force of gravity by the hydraulic drive, that the absorber rods also remain thereafter safely in the inserted position in the core. For this reason, the spindle nuts are made to follow up the hydraulic drive, which in addition to providing the possibility to bring a rod which is not quite inserted, into its fully inserted position in the core, then additionally holds the rods in their end positions, fully inserted or against unintended sliding down.
The threaded spindles of the absorber rod mechanical drives, which are driven as a rule by electric motors independent of hydraulic power, are therefore important not only from an operating but also from a safety point of view. For the further development of reactor safety, it is of particular advantage that, in addition to the fast shut-off system provided by the hydraulic drives, the mechanical spindle drives in conjunction with a recovery of the pumping power of the coolant forced-circulation pumps, constitute a second reactor shutdown system, which is sufficiently effective for certain transient processes.
One object of the invention is therefore to create a mechanical rod-positioning drive, in which the threaded spindle has a screw thread pitch as large as possible and can safely also be run, if necessary, with high speed of rotation for a core shutdown, so the absorber rod can be run into the reactor core faster than is otherwise customary for control or normal adjustment, so that in this way the mechanical drive can also take over the scram function, if necessary.
It is therefore the purpose of the present invention to improve the engagement of the spindle or traveling nut with the threads of the spindle in such a manner that relative velocities between the spindle and the traveling nut can be achieved which have not been possible heretofore. As already mentioned, it is possible with such high relative velocities, which lead to fast motion of the absorber rods through the use of a high speed of rotation or a particularly large pitch of the spindle threads, to achieve the fast motion of the absorber rods necessary for instance, for a fast shutdown of the nuclear reactor, without having to cancel the engagement between the rollers and the threads of the spindle.
In boiling-water reactors, it is further of advantage that the mechanical spindle drive, as was proposed, for instance, in the German Offenlegungsschriften Nos. 1,950,646 and 2,007,771, be located in the interior of the reactor pressure vessel, so that the bottom of the vessel can be designed with smaller drive hosuing stubs, into which smaller drive housings can then be welded which also, however, may as well be omitted.
The safety of the reactor can be further increased also if the spindle diameter and therefore, the drive housings and their stub feedthroughs, can be made smaller.
It is therefore also an object of the invention to create a spindle mechanism which is capable of meeting the safety requirements and high availability specifications even better than has been the case up to now, with the drives mounted outside as well as inside the pressure vessel. This applies particularly to control rod drives which are located inside the reactor pressure vessel and likewise have spindle drives which, however, must run there, continuously in hot reactor water (moderator) of about 300.degree. C.
Up to now, the last-mentioned threaded spindles, which are internal to the pressure vessel, have not yet been in use under such more difficult and broader conditions. The situation is different with the spindle drives for boiling-water reactors which are already being used and are disposed in control rod drives which are situated outside the reactor pressure vessel and run in reactor water with considerably lower temperatures and relatively lower velocities.
Accordingly, spindle drives for adjusting the absorber rods are known and in use in nuclear power plants, in which the spindle nut is supported in roller elements (revolving-ball spindles) and such, in which they are guided with sliding friction and where the sliding surfaces may consist of steel and graphite.
In the case of sliding friction, particularly in aggressive reactor water, there is a pronounced danger of destruction of sliding surfaces by corrosion, especially if a potential difference between the two running partners prevails, whereby the destruction of the running surfaces by chemical corrosion is initiated.
This can be circumvented by unequal running partners, e.g., steel spindles and plastic nuts. These, however, are highly imperiled in the long run due to wear and loss of strength, particularly if used in hot water, so that graphite has so far turned out as one of the few possibilities for the nut material.
However, the graphite nut, too, loses its strength in the long run, particularly due to wear and shock stresses. Their use therefore necessitates relatively large spindle diameters and support in springs. It is not usable for axial or rod-travel velocities of more than 4 mm/sec.
The functioning of the revolving-ball spindle is jeopardized in the case of large pitch, if the absorber rod drive is located inside the reactor pressure vessel. In spite of rolling friction, where to a small extent, sliding friction is also present because of the relative motion between the balls and the ball races, this spindle type is in danger in hot reactor water due to the same, above-mentioned chemical-mechanical corrosion effect.
The formation of oxide layers at the running surfaces and the possibility that dirt from the cooling water can come in, lead in time to a blocking of the revolving balls which are so important for the proper functioning of such a drive. If one wants to counter this danger by ample radial and axial play between the nut and the spindle, then there is the danger of interference with the operation due to ball eccentricity (whip) and vibrations resulting therefrom, whereby the surfaces of the balls and therefore also the wiper tabs at the rod return tubes, can be damaged due to excessive contact pressure. Fitting the nut without play, such as tensioning two half-nuts against each other, also does not solve the problem because, although otherwise the conditions may be more favorable, the unimpeded circulation of the balls is jeopardized by additional clamping forces, particularly with a large spindle thread pitch.
It is therefore a further object of the invention to eliminate these disadvantages and to create a mechanical spindle drive which meets the above mentioned stricter safety requirements and permits, beyond this, also high operating availability. The latter is also important, as due to the safety philosophy, operating trouble in one or two absorber rod drives already may lead to a shutdown of the nuclear power plant.
The subject of the invention is now a positioning drive for absorber rods of a nuclear reactor, particularly a boiling-water reactor, with a spindle and a traveling nut which comprises anti-friction bearing means which engage the threads of the spindle.